An unfailing observation over the past 70 years is that resistance to all antibiotics emerges eventually after use in the clinic. Where does this resistance come from? Recent work has shown that antibiotic resistance genes are common in metagenomes of ancient sediments. This prevalence of resistance, well before the use of antibiotics, denotes the importance of taking microbial chemical ecology and deep metagenomic profiling into account in the development and use of antibiotics.